1. Field of the Invention
This invention relates generally to the field of seismic prospecting and more particularly to an impact-type seismic source employing a repetitively cycled mass-piston powered by a closed air system.
2. Description of the Prior Art
U.S. Pat. No. 4,284,164 issued Aug. 18, 1981, to Tom P. Airhart entitled "Acoustic Pulse Generator" discloses apparatus for generating an acoustic pulse in a medium, the contents of which patent are hereby incorporated herein by reference. The apparatus includes an upstanding hollow cylindrical housing within which an impact piston is suspended. Upon issuance of a firing signal, the piston is driven downwardly by compressed air to strike a target plate positioned on the earth. The compressed air is provided by accumulators or the like which communicate with the bore of the housing near its upper end through ports in the housing sidewall. In its upper most position, the sidewall of the piston blocks these ports and is provided with piston rings which isolate the top of the piston from leakage of high pressure air. When the piston is displaced downwardly in a firing operation, it unblocks the pressure ports. High pressure from the accumulators is now exposed to the top of the piston and aids gravity in propelling it downwardly to strike the target plate. In the system described the top of the housing is provided with a vent communicating with the bore above the piston. The purpose of this vent is to provide an upward path for air escape during a firing operation so as to compensate for recoil force on the housing. Thus, the system is not a closed air system and the accumulators must be of large volume and periodically recharged.
In commonly assigned co-pending U.S. patent application, Ser. No. 455,172, filed Jan. 3, 1983, in the name of Andrew B. Woodrow, an impact-type seismic source similar in function to that described above is disclosed which, however, does employ a closed air system which is self-pressurizing between shots. In accordance with one embodiment of the modified apparatus of such application, the space in the housing above the piston is no longer constantly vented to atmosphere, but only when a valve is opened in a conduit leading into such space. As in the apparatus of U.S. Pat. No. 4,204,164, air enters the housing from the pressure vessels as the piston is driven downwardly during a firing operation. But in contrast to the patented apparatus, when the piston returns, air is repressurized into the pressure vessels at substantially the original pressure. In such a system, however, it is apparent that some means must be provided for relieving pressure build up in the thin air layer remaining above the piston once the pressure vessels are sealed off by blockage of the ports. Typically, the pressure vessels are charged to a value on the order of 300 p.s.i. Without some relief, the air trapped in the confined space above the piston between firings may easily be compressed to 700 or 800 p.s.i. If allowed to remain until the next firing of the piston this trapped pressure would impose a heavy strain on any support mechanism such as an electrical latch. But even without failure of such support mechanism, this trapped pressure could interfere with the intended firing cycle by changing the timing of the intended seismic impact in relation to an electrical firing signal. This would be particularly objectionable where multiple sources are to be fired simultaneously in order to enhance signal strenght.
While the external valve mechanism described in the referenced application relieves such trapped pressure, it has disadvantages. If it is solenoid operated, the solenoid may stick under certain weather conditions. Furthermore, any electrical system necessary in order to trigger the operation of such solenoid valves adds to complexity and increases risk of malfunction.
A conventional pressure activated relief valve at the top of the actuator housing might appear to eliminate the need for external electrical controls. However, this would be unworkable in the present system for several reasons. The relief valve would have to be set to open above the operating pressure of the actuator in order not to open prematurely during firing and thus degrade the strength of the downward thrust against the piston. However, upon return of the piston to its upper most position, such a relief valve would remain open only so long as such excess pressure were present within the space above the piston. In otherwords, some high pressure air would remain trapped in the bore of the actuator between firing with at least some of the undesirable results alluded to above. What is needed, therefore, is an improved valving arrangement which completely circumvents the possibility of any such pressure build up.